On Mon, May 07, 2018 at 02:45:14PM -0700, Joel Fernandes wrote:
> On Mon, May 07, 2018 at 02:08:01PM -0700, Paul E. McKenney wrote:
> > On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote:
> > > From: "Joel Fernandes (Google)" <[email protected]>
> > >
> > > In recent tests with IRQ on/off tracepoints, a large performance
> > > overhead ~10% is noticed when running hackbench. This is root caused to
> > > calls to rcu_irq_enter_irqson and rcu_irq_exit_irqson from the
> > > tracepoint code. Following a long discussion on the list [1] about this,
> > > we concluded that srcu is a better alternative for use during rcu idle.
> > > Although it does involve extra barriers, its lighter than the sched-rcu
> > > version which has to do additional RCU calls to notify RCU idle about
> > > entry into RCU sections.
> > >
> > > In this patch, we change the underlying implementation of the
> > > trace_*_rcuidle API to use SRCU. This has shown to improve performance
> > > alot for the high frequency irq enable/disable tracepoints.
> [...]
> >> Signed-off-by: Joel Fernandes (Google) <[email protected]>
> > > ---
> > > include/linux/tracepoint.h | 46 +++++++++++++++++++++++++++++++-------
> > > kernel/tracepoint.c | 15 ++++++++++++-
> > > 2 files changed, 52 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > > index c94f466d57ef..f56f290cf8eb 100644
> > > --- a/include/linux/tracepoint.h
> > > +++ b/include/linux/tracepoint.h
> > > @@ -15,6 +15,7 @@
> > > */
> > >
> > > #include <linux/smp.h>
> > > +#include <linux/srcu.h>
> > > #include <linux/errno.h>
> > > #include <linux/types.h>
> > > #include <linux/cpumask.h>
> > > @@ -33,6 +34,8 @@ struct trace_eval_map {
> > >
> > > #define TRACEPOINT_DEFAULT_PRIO 10
> > >
> > > +extern struct srcu_struct tracepoint_srcu;
> > > +
> > > extern int
> > > tracepoint_probe_register(struct tracepoint *tp, void *probe, void
> > > *data);
> > > extern int
> > > @@ -77,6 +80,9 @@ int unregister_tracepoint_module_notifier(struct
> > > notifier_block *nb)
> > > */
> > > static inline void tracepoint_synchronize_unregister(void)
> > > {
> > > +#ifdef CONFIG_TRACEPOINTS
> > > + synchronize_srcu(&tracepoint_srcu);
> > > +#endif
> > > synchronize_sched();
> > > }
> > >
> > > @@ -129,18 +135,38 @@ extern void syscall_unregfunc(void);
> > > * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
> > > * "void *data", where as the DECLARE_TRACE() will pass in "void *data,
> > > proto".
> > > */
> > > -#define __DO_TRACE(tp, proto, args, cond, rcucheck)
> > > \
> > > +#define __DO_TRACE(tp, proto, args, cond, rcuidle)
> > > \
> > > do { \
> > > struct tracepoint_func *it_func_ptr; \
> > > void *it_func; \
> > > void *__data; \
> > > + int __maybe_unused idx = 0; \
> > > \
> > > if (!(cond)) \
> > > return; \
> > > - if (rcucheck) \
> > > - rcu_irq_enter_irqson(); \
> > > - rcu_read_lock_sched_notrace(); \
> > > - it_func_ptr = rcu_dereference_sched((tp)->funcs); \
> > > + \
> > > + /* \
> > > + * For rcuidle callers, use srcu since sched-rcu \
> > > + * doesn't work from the idle path. \
> > > + */ \
> > > + if (rcuidle) { \
> > > + if (in_nmi()) { \
> > > + WARN_ON_ONCE(1); \
> > > + return; /* no srcu from nmi */ \
> > > + } \
> > > + \
> > > + idx = srcu_read_lock_notrace(&tracepoint_srcu); \
> > > + it_func_ptr = \
> > > + srcu_dereference_notrace((tp)->funcs, \
> > > + &tracepoint_srcu); \
> > > + /* To keep it consistent with !rcuidle path */ \
> > > + preempt_disable_notrace(); \
> > > + } else { \
> > > + rcu_read_lock_sched_notrace(); \
> > > + it_func_ptr = \
> > > + rcu_dereference_sched((tp)->funcs); \
> > > + } \
> > > + \
> > > if (it_func_ptr) { \
> > > do { \
> > > it_func = (it_func_ptr)->func; \
> > > @@ -148,9 +174,13 @@ extern void syscall_unregfunc(void);
> > > ((void(*)(proto))(it_func))(args); \
> > > } while ((++it_func_ptr)->func); \
> > > } \
> > > - rcu_read_unlock_sched_notrace(); \
> > > - if (rcucheck) \
> > > - rcu_irq_exit_irqson(); \
> > > + \
> > > + if (rcuidle) { \
> >
> > Don't we also need an in_nmi() check here in order to avoid unbalanced
> > srcu_read_unlock_notrace() calls?
>
> The in_nmi() in the lock path should take care of making sure its balanced.
>
> The diff the way its formatted appears confusing as Mathieu pointed.
Ah, right, I was for some reason thinking that the two hunks of the
diff were two separate macros. Apologies for my confusion!
Thanx, Paul
